Re: [PATCH v2 01/16] powerpc: Replace unreachable() with it's builtin variant in __WARN_FLAGS()

[Naveen N. Rao]

Christophe Leroy wrote:
> Le 29/08/2022 à 07:52, Sathvika Vasireddy a écrit :
> > objtool is throwing *unannotated intra-function call* warnings in
> > .c files with a few instructions that are marked unreachable. The
> > problem comes from the annotate_unreachable() macro that is
> > called by unreachable(). This annotation is adding a call to a
> > function with size 0, and objtool does not add such symbols
> > to the rbtree. Due to this reason, find_call_destination() function
> > is not able to find the destination symbol for that call.
> >
> > With the annotation (annotate_unreachable()), gcc seems to
> > generate a 'bl' to unreachable symbol with size 0. But with
> > the builtin variant of unreachable (__builtin_unreachable()),
> > gcc does not emit calls to such symbols and the warnings
> > go away. Given that the codegen remains same, and that
> > there are no 'bl' instructions to such symbols emitted, fix
> > these warnings by replacing unreachable() with it's builtin
> > variant in __WARN_FLAGS().
>
> How can you say that the codegen remains the same if with the original 
> you get stale 'bl' instructions and with the alternative you don't ?

I guess the reference to codegen remaining the same is more to do with 
unreachable vs. __builtin_unreachable() in the absence of 
CONFIG_OBJTOOL. But yeah, the changelog needs to be reworked to clarify 
that.

> > Also, add barrier_before_unreachable() before __builtin_unreachable()
> > to work around a gcc bug [1], for the problem reported at [2].
>
> Here my comment was not related to the gcc bug [1] but to gcc bug 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365 , which was worked 
> around by commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()")
>
> By chance it also solve the problem [1] as you mention.

That's a good commit to reference, but please also retain a link to the 
new PR.


- Naveen